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Water Pollution: Types, Sources, Effects and Control (4274 Words)

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Read this article to learn about the types, sources, effects and control of water pollution!

Water pollution is the contamination of water bodies (e.g. lakes, rivers, oceans and groundwater). Water pollution affects plants and organisms living in these bodies of water and in almost all cases the effect is damaging not only to individual species and populations, but also to the natural communities.

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Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds. Point source pollution refers to contaminants that enter a waterway through a discrete conveyance, such as a pipe or ditch. Examples of sources in this category include discharges from a sewage treatment plant, a factory, or a city storm drain.

Non-point source pollution: Non-point source (NPS) pollution refers to diffuse contamination that does not originate from a single discrete source. NPS pollution is often the cumulative effect of small amounts of contaminants gathered from a large area. The leaching out of nitrogen compounds from agricultural land which has been fertilized is a typical example. Nutrient runoff in storm water from “sheet flow” over an agricultural field or a forest are also cited as examples of NPS pollution.

Contaminated storm water washed off of parking lots, roads and highways, called urban runoff, is sometimes included under the category of NPS pollution. However, this runoff is typically channelled into storm drain systems and discharged through pipes to local surface waters, and is a point source. However where such water is not channelled and drains directly to ground it is a non-point source.

Types of Water Pollution:

There are two types of water pollution: Ground water pollution & Surface water pollution

i. Groundwater Pollution:

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Considerable amount of Earth’s water is found in soil or under rock structures called aquifers. People use aquifers to obtain drinking water and build wells to access it. In case this water becomes polluted, it is called groundwater pollution. This is caused by pesticide contamination from the soil and this can infect the drinking water and lead to huge problems.

Groundwater refers to water collected under the Earth’s surface. The sources of groundwater are rain, snow, hail, sleet, etc. Water that falls on the Earth’s surface continues to travel downwards due to gravity, until a zone comes where it is saturated with water.

At this depth, the spaces between the soil and rock particles are filled up with water. This particular zone is known as the saturated zone. The topmost portion of the saturated zone is referred to as water table. The level of water table changes depending upon the season, it is highest in spring and lowest in summer.

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Groundwater is connected to surface water such as rivers, streams and lakes. In fact, there is continuous exchange of water between surface water and groundwater. Groundwater pollution is a change in the properties of groundwater due to contamination by microbes, chemicals, hazardous substances and other foreign particles. It is a major type of water pollution. The sources of groundwater pollution are either natural (mineral deposits in rocks) or man-made.

Natural sources are less harmful compared to hazardous chemicals generated by human activities. Any chemical present on the surface can travel underground and cause groundwater pollution. The seepage of the chemical depends on the chemical type, soil porosity and hydrology.

One of the major sources of groundwater pollution is industries. Manufacturing and other chemical industries require water for processing and cleaning purposes. This used water is recycled back to water sources without proper treatment, which in turn, results in groundwater pollution.

It is also to be noted that solid industrial wastes that are dumped in certain areas also contribute to groundwater pollution. When rainwater seeps downwards, it dissolves some of these harmful substances and contaminates groundwater.

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Another source of groundwater pollution is agriculture; the fertilizers, pesticide and other chemicals used in growing plants contaminate groundwater. Residential areas also generate pollutants (microorganisms and organic compounds) for groundwater contamination.

Groundwater pollutant can be divided into point source and non-point source based on the nature of disposal. The former refers to contaminants originating from a particular source such as sewage pipe or tank; whereas non-point source is spread over large areas (for example, pesticides and fertilizers).

Groundwater pollution cannot be prevented completely. As there are varied sources, it is not always practical to prevent the contamination of groundwater. However, there is no doubt that individuals can contribute in many ways to reduce groundwater pollution.

Some of the basic tips are proper disposal of waste, waterproof storage of household chemicals (paints, medicines and detergents) and agricultural chemicals to avoid leaching, etc. Proper installation of septic systems along with regular cleaning will reduce groundwater contamination.

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It is very difficult and costly to treat contaminated groundwater. Hence, it is better to minimize the risk of groundwater pollution. Public awareness programs about the importance of groundwater and ways to minimize its contamination should be implemented.

ii. Surface Water Pollution:

These are the natural water resources of the Earth. These are found on the exterior of the Earth’s crust, oceans, rivers and lakes.

Water is an essential commodity for survival. We need water for drinking, cooking, bathing, washing, irrigation, and for industrial operations. Most of the water for such uses comes from rivers, lakes or groundwater sources. Water has the property to dissolve many substances in it, therefore, it can easily get polluted.

Pollution of water can be caused by “point sources” or “non- point sources”. Point sources are specific sites near water which directly discharge effluents into them. Major point sources of water pollution are industries, power plants, underground coal mines, offshore oil wells etc.

The discharge from non-point sources is not at any particular site, rather, these sources are scattered, which individually or collectively pollute water. Surface run-off from agricultural fields, overflowing small drains, rain water sweeping roads and fields, atmospheric deposition etc., are the non-point sources of water pollution.

Sources of Surface Water Pollution:

1. Sewage:

Emptying the drains and sewers in fresh water bodies causes water pollution. The problem is severe in cities.

Waste heat from industrial discharges increases the temperature of water bodies and affects distribution and survival of sensitive species.

Sources and Types of Water Pollutants:

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Pathogens:

Coliform bacteria are a commonly used bacterial indicator of water pollution, although not an actual cause of disease. Other microorganisms sometimes found in surface waters which have caused human health problems include:

i. Burkholderia pseudomallei.

ii. Cryptosporidium parvum.

iii. Giardia lamblia.

iv. Salmonella.

v. Novovirus and other viruses.

vi. Parasitic worms (helminths).

High levels of pathogens may result from inadequately treated sewage discharges. This can be caused by a sewage plant designed with less than secondary treatment (more typical in less- developed countries. In developed countries, older cities with aging infrastructure may have leaky sewage collection systems (pipes, pumps, valves), which can cause sanitary sewer overflows. Some cities also have combined sewers, which may discharge untreated sewage during rain storms. Pathogen discharges may also be caused by poorly managed livestock operations.

Chemical and other contaminants: Contaminants may include organic and inorganic substances.

Organic water pollutants include:

i. Detergents.

ii. Disinfection by-products found in chemically disinfected drinking water, such as chloroform.

iii. Food processing waste, which can include oxygen-demanding substances, fats and grease.

iv. Insecticides and herbicides, a huge range of organohalides and other chemical compounds.

iii. Fertilizers having a large quantity of nitrogen and phosphorus cause a high biological oxygen demand in the water. The high amount of BOD is responsible for oxygen depletion in water bodies.

iv. Human settlement along the banks of rivers causes human, animal and industrial waste to be discharged into it.

Effect of Water Pollution:

i. Disorders:

Some pollutants like sodium can cause cardiovascular diseases, while mercury and lead cause nervous disorders.

ii. Toxic Substances:

DDT is toxic material which can cause chromosomal changes. Some of these substances like pesticides, methyl mercury etc., move into the bodies of organisms from the medium in which these organisms live. These substances tend to accumulate in the organism’s body from the medium food. This process is called bioaccumulation or bio concentration. The concentration of these toxic substances builds up at successive levels of food chain. This process is called bio magnifications.

iii. Water Pollution:

Fluoride pollution causes defects in teeth and bones, a disease called fluorosis while arsenic can cause significant damage to the liver and the nervous system. In addition to all these, organic compounds present in the polluted water facilitate the growth of algae and other weeds, which in turn use more oxygen dissolved in the water. This reduces the amount of oxygen dissolved in the water and the consequent shortage of oxygen for other aquatic life.

iv. Asbestos:

This pollutant is a serious health hazard and carcinogenic. Asbestos fibers can be inhaled and cause illnesses such as asbestosis, mesothelioma, lung cancer, intestinal cancer and liver cancer.

v. Mercury:

This is a metallic element and can cause health and environmental problems. It is a non-biodegradable substance so is hard to clean up once the environment is contaminated. Mercury is also harmful to animal health as it can cause illness through mercury poisoning.

vi. Phosphates:

The increased use of fertilizers means that phosphates are more often being washed from the soil and into rivers and lakes. This can cause eutrophication, which can be very problematic to marine environments.

vii. Oils:

Oil does not dissolve in water; instead it forms a thick layer on the water surface. This can stop marine plants receiving enough light for photosynthesis. It is also harmful for fish and marine birds.

viii. Petrochemicals:

This is formed from gas or petrol and can be toxic to marine life.

ix. Organic matter which reaches water bodies is decomposed by micro-organisms present in water. For this degradation, oxygen dissolved in water is consumed. Dissolved oxygen (DO) is the amount of oxygen dissolved in a given quantity of water at a particular temperature and atmospheric pressure.

Amount of dissolved oxygen depends on aeration, photosynthetic activity in water, respiration of animals and plants and ambient temperature.The saturation value of DO varies from 8-15 mg/L. For active fish species (trout and Salmon) 5-8 mg/L of DO is required whereas less desirable species like carp can survive at 3.0 mg/L of DO.Lower DO may be harmful to animals especially fish population. Oxygen depletion (deoxygenating) helps in release of phosphates from bottom sediments and causes eutrophication.

x. Addition of compounds containing nitrogen and phosphorus helps in the growth of algae and other plants which when die and decay consume oxygen of water. Under anaerobic conditions foul smelling gases are produced. Excess growth or decomposition of plant material will change the concentration of CO2 which will further change pH of water. Changes in pH, oxygen and temperature will change many physicochemical characteristics of water.

xi. Lead in water may be released from water pipes as lead is used in plumbing. Lead poisoning affects kidneys reproductive system, liver, brain and central nervous system. It also causes anaemia and mental retardation in children.

xii. Nitrate ions present in the water are harmful to human health. From nitrogen fertilizers, nitrate ions seep into water bodies from where these may bioaccumulate in the bodies of the consumers. In the stomach nitrate is reduced to nitrite and is responsible for blue baby syndrome and stomach cancer.

Control of Water Pollution:

The following points may help in reducing water pollution from non-point sources.

(i) Judicious use of agrochemicals like pesticides and fertilizers which will reduce their surface run-off and leaching. Use of these on sloped lands should be avoided.

(ii) Use of nitrogen fixing plants to supplement the use of fertilizers.

(iv) Prevent run-off of manure. Divert such run-off to basin for settlement. The nutrient rich water can be used as fertilizer in the fields.

(v) Separate drainage of sewage and rain water should be provided to prevent overflow of sewage with rain water.

(vi) Planting trees would reduce pollution by sediments and will also prevent soil erosion.

For controlling water pollution from point sources, treatment of waste waters is essential before being discharged. Parameters which are considered for reduction in such water are: Total solids, biological oxygen demand (BOD), chemical oxygen demand (COD), nitrates and phosphates, oil and grease, toxic metals etc. Waste waters should be properly treated by primary and secondary treatments to reduce the BOD, COD levels up to the permissible levels for discharge.

Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally-safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertilizer).

Sewage is created by residential, institutional, and commercial and industrial establishments and includes household waste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry.

Sewage can be treated close to where it is created (in septic tanks, bio-fitter’s or aerobic treatment systems), or collected and transported via a network of pipes and pump stations to a municipal treatment plant.

Conventional sewage treatment may involve three stages, called primary, secondary and tertiary treatment. Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.

Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro­organisms from the treated water prior to discharge or tertiary treatment.

Tertiary treatment is sometimes defined as anything more than primary and secondary treatment. Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

i. Pre-treatment removes materials that can be easily collected from the raw wastewater before they damage or clog the pumps and skimmers of primary treatment clarifiers (trash, tree limbs, leaves, etc.).

ii. Screening:

The influent sewage water is screened to remove all large objects carried in the sewage stream. This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants a manually cleaned screen may be used.

The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed they become entrained in pipes and moving parts.

iii. Grit removal:

Pre-treatment may include a sand or grit channel or chamber where the velocity of the incoming wastewater is adjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is desirable at larger plants.

iv. Fat and grease removal:

Fat and grease is removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as froth. In most plants however, fat and grease removal takes place in the primary settlement tank using mechanical surface skimmers.

v. Primary treatment:

In the primary sedimentation stage, sewage flows through large tanks, commonly called “primary clarifiers” or “primary sedimentation tanks.” The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off.

Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Grease and oil from the floating material can sometimes be recovered for saponification.

The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A typical sedimentation tank may remove from 60 to 65 per cent of suspended solids, and from 30 to 35 per cent of biochemical oxygen demand (BOD) from the sewage.

vi. Secondary treatment:

Secondary treatment is designed to substantially degrade the biological content of the sewage which is derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota requires both oxygen and food to live.

Fixed-film or attached growth systems include trickling filters and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface. Suspended-growth systems include activated sludge, where the biomass is mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water.

However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.

vii. Rotating biological contactors:

Rotating biological contactors (RBCs) are mechanical secondary treatment systems, which are robust and capable of withstanding surges in organic load. RBCs were first installed in Germany in 1960 and have since been developed and refined into a reliable operating unit.

The rotating disks support the growth of bacteria and micro-organisms present in the sewage, which break down and stabilize organic pollutants. To be successful, micro-organisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage.

Effluent from the RBC is then passed through final clarifiers where the micro-organisms in suspension settle as sludge. The sludge is withdrawn from the clarifier for further treatment. A functionally similar biological filtering system has become popular as part of home aquarium filtration and purification.

The aquarium water is drawn up out of the tank and then cascaded over a freely spinning corrugate fiber-mesh wheel before passing through a media filter and back into the aquarium. The spinning mesh wheel develops a biofilm coating of microorganisms that feed on the suspended wastes in the aquarium water and are also exposed to the atmosphere as the wheel rotates. This is especially good at removing waste urea and ammonia urinated into the aquarium water by the fish and other animals.

viii. Biological aerated filters:

The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia (nitrification) to nitrate, followed by de-nitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria.

The oxidation of ammonia (NH3) to nitrite (NO3) is most often facilitated by Nitrosomonas spp. (nitroso referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp.De-nitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria.

ix. Secondary sedimentation:

The final step in the secondary treatment stage is to settle out the biological floe or filter material through a secondary clarifier and to produce sewage water containing low levels of organic material and suspended matter.

x. Tertiary treatment:

The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called “effluent polishing.”

xii. Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates.

Sludge treatment & disposal:

i. Anaerobic digestion:

Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a temperature of around 36°C.

Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge. Anaerobic digestion is the most common (mesophilic) treatment of domestic sewage in septic tanks, which normally retain the sewage from one day to two days, reducing the BOD by about 35 to 40 percent.

This reduction can be increased with a combination of anaerobic and aerobic treatment by installing Aerobic Treatment Units (ATUs) in the septic tank. One major feature of anaerobic digestion is the production of biogas (with the most useful component being methane), which can be used in generators for electricity production and/or in boilers for heating purposes.

ii. Aerobic digestion:

Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The operating costs used to be characteristically much greater for aerobic digestion because of the energy used by the blowers, pumps and motors needed to add oxygen to the process. Aerobic digestion can also be achieved by using diffuser systems or jet aerators to oxidize the sludge.

iii. Composting:

Composting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.

iv. Incineration:

Incineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural gases or fuel oil) required to burn the low calorific value sludge and vaporize residual water.

Stepped multiple hearth incinerators with high residence time and fluidized bed incinerators are the most common systems used to combust wastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, this option being less expensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.

v. Sludge disposal:

When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically, sludge’s are thickened (dewatered) to reduce the volumes transported off-site for disposal.

There is no process which completely eliminates the need to dispose of bio-solids. There is, however, an additional step some cities are taking to superheat sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials.